Medical ultrasound is extensively used for studying flow dynamics in the human body by using color flow mapping. The technique displays a color image of the flow superimposed on the normal anatomic B-mode image. The velocity component along the ultrasound beam direction is measured, and a flow transverse to the beam is not displayed. This is shown in FIG. 1, where the flow in the carotid artery and the jugular vein is displayed. The image is acquired with a convex array, and the angles between flow direction and the ultrasound beam changes over the image. Notice the chance of estimated flow direction around the dashed line in both vessels due to the change of angle between the flow and the ultrasound beam. This is one of the main limitations of current ultrasound flow systems, since most vessels are parallel to the skin surface, and therefore it is a problem to get a sufficiently small angle between the flow and the beam. Also the flow is often not parallel to the vessel surface, and it is therefore difficult, if not impossible, to estimate the correct angle and compensate for it [1].
Several authors have attempted to remedy this artifact. Fox [2] suggested using two beams to find the transverse component. The system works well for large transducers and investigations close to the transducer, but the variance of the transverse component increases for situations with large depths and smaller transducers as used in cardiac scanning through the ribs. Trahey and co-workers [3] have suggested using speckle tracking in which a small search region in one image is correlated or compared to a subsequent image. This approach has problems in terms of frame rate, since images are compared, and the resolution of the velocity estimates can be low. Newhouse et al [4] developed a method in which the total bandwidth of the received signal is affected by the transverse velocity. It is, however, often difficult to find this bandwidth due to the inherent noise in the signal.
In this invention a new and improved estimator is presented for the approach described previously in [5] and [6], which makes it possible to estimate the flow vector using a transversally modulated probing field.